CDMA systems employ wide-band signals with good cross-correlation properties [Kohno, Meidan, and Milstein, 1995]. That means, the output of a filter matched to one user's signal is small when a different user's signal is input. A large body of work exists on spreading sequences which lead to signal sets with small cross-correlations [Sarwate and Pursley, 1980]. Because of their noise-like appearance such sequences are often referred to as pseudo-noise (PN) sequences and because of their wide-band nature CDMA systems are often called spread-spectrum systems.
Spectrum spreading can be achieved mainly in two ways: through frequency hopping as explained above or through direct sequence spreading. In direct sequence spread spectrum a high-rate, antipodal pseudo-random spreading sequence modulates the transmitted signal such that the bandwidth of the resulting signal is roughly equal to the rate of the spreading sequence. The cross-correlation of the signals is then largely determined by the cross-correlation properties of the spreading signals. Clearly, CDMA signals overlap in both time and frequency domain but are separable based on their spreading waveforms.
An immediate consequence of this observation is that CDMA systems do not require tight synchronization like TDMA systems. By the same token, frequency planning and management are not required as frequencies are re-used throughout the coverage area.
